Inflammatory bowel diseases are chronic relapsing conditions caused by dysregulated immune response to commensal nonpathogenic enteric bacteria in genetically susceptible individuals. We use gnotobiotic rodent models and in vitro modeling to understand the contributions of both bacterial and host immune factors in the pathogenesis of chronic, immune-mediated intestinal inflammation. Germ-free (GF, sterile) genetically engineered rats and mice fail to develop colitis and pathogenic immune responses, which are initiated almost immediately after colonizing susceptible hosts with commensal bacteria. Our results in HLA B27/ (2 microglobulin transgenic (TG) rats show that Bacteroides vulgatus, but not B. distasonis, E. coli and many other bacterial species cause T cell-mediated colitis. However, B. vulgatus does not cause disease in normal, wild type (WT) rodents, which develop tolerogenic immune responses that allow the hosts to exist in a hostile microbial environment. We have identified B. vulgatus heat shock protein 60 (HSP 60) as an immunodominant antigen in monoassociated HLA B27 TG rats and identified 3 novel antigens that activate T eel Is from colitic rats in a metagenomic library created from colonic contents of SPF HLA B27 TG rats with colitis. Hypotheses: 1. Chronic colitis in genetically susceptible hosts is caused by dysregulated cell-mediated immune responses to defined bacterial antigens unique to each individual host. Autoimmune responses do not cause colitis. 2. Tolerance in normal hosts is mediated by carefully regulated interactions between innate and acquired immune responses that inhibit pathogenic immune activation by commensal enteric bacteria. Specific Aims: 1: Identify dominant antigens from B. vulgatus and the com mensal microbiota that induce immune-mediated colitis in susceptible hosts. We will determine the ability of genetically engineered bacteria expressing or lacking HSP 60 to induce colitis in monoassociated T G rats and explore the ability of novel bacterial antigens to stimulate pathogenic immune responses and cause colitis in TG rats. 2. Determine bacterial antigen specificity in T cell-mediated colitis. We will examine molecular mimicry between host and bacterial antigens and between different bacterial species by transferring CD4+ T cells to nude HLA B27 TG rats while controlling the bacterial species in both donor and recipient gnotobiotic rats. 3. Identify mechanisms by which luminal bacteria induce tolerance in normals and chronic immune activation in susceptible hosts. We will explore mechanisms of paracrine regulation of bacterial-induced NF B activation in epithelial cells by co-cultured mononuclear cells studies and the role of T and B cells to regulate in vivo NF B activation and innate immune responses in the epithelium of monoassociated rats. These innovative, mechanistic studies take advantage of our unique gnotobiotic rodent facilities, a proven multidisciplinary investigative team, and novel preliminary data.